Recently, a driving circuit of a display device, a personal computer or the like includes a semiconductor device such as a transistor, a diode or the like as a microscopic switching element. Especially in a display device, a semiconductor device is used as a selective transistor that supplies a voltage or a current in accordance with the gray scale of each of pixels and also used in a driving circuit that selects a pixel to which the voltage or the current is to be supplied. The characteristics required of a semiconductor vary in accordance with the use thereof. For example, a semiconductor used as a selective transistor is required to have a low off-current or little variance in characteristics from another selective semiconductor included in the same device. A semiconductor used in a driving circuit is required to have a high on-current.
To be used in a display device as described above, a semiconductor device including a channel formed of amorphous silicon, low-temperature polysilicon or single crystalline silicon has been conventionally developed. A semiconductor device including a channel formed of amorphous silicon can be formed with a simpler structure and in a process of 400° C. or lower, and therefore can be formed, for example, by use of a large glass substrate referred to as an eighth-generation glass substrate (2160×2460 mm). However, such a semiconductor device including a channel formed of amorphous silicon has a low mobility and is not usable in a driving circuit.
A semiconductor device including a channel formed of low-temperature polysilicon or single crystalline silicon has a higher mobility than the semiconductor device including a channel formed of amorphous silicon, and therefore is usable as a selective transistor and also in a driving circuit. However, such a semiconductor device including a channel formed of low-temperature polysilicon or single crystalline silicon has a complicated structure and needs a complicated process to be manufactured. In addition, such a semiconductor device needs to be formed in a process of 500° C. or higher, and therefore cannot be formed by use of a large glass substrate as described above. A semiconductor device including a channel formed of amorphous silicon, low-temperature polysilicon or single crystalline silicon has a high off-current. In the case where such a semiconductor device is used as a selective transistor, it is difficult to keep the applied voltage for a long time.
For the above-described reasons, a semiconductor device including a channel formed of an oxide semiconductor, instead of amorphous silicon, low-temperature polysilicon or single crystalline silicon, has been progressively developed recently (e.g., Japanese Laid-Open Patent Publication No. 2010-062229 and Japanese Laid-Open Patent Publication No. 2014-194579). It is known that a semiconductor device including a channel formed of an oxide semiconductor can be formed with a simple structure and in a process of 400° C. or lower like a semiconductor device including a channel formed of amorphous silicon, and has a mobility higher than that of a semiconductor device including a channel formed of amorphous silicon. It is also known that such a semiconductor device including a channel formed of an oxide semiconductor has a very low off-current.
However, the mobility of the semiconductor device including a channel formed of an oxide semiconductor is lower than that of the semiconductor device including a channel formed of low-temperature polysilicon or single crystalline silicon. Therefore, in order to provide a higher on-current, the semiconductor device including a channel formed of an oxide semiconductor needs to have a shorter L length (channel length). In order to shorten the channel length of the semiconductor device described in Japanese Laid-Open Patent Publication No. 2010-062229 and Japanese Laid-Open Patent Publication No. 2014-194579, a distance between a source and a drain needs to be shortened.
The distance between a source and a drain is determined by a photolithography step and an etching step. In the case where patterning is performed by photolithography, size reduction is restricted by the size of a mask pattern of an exposure device. Especially in the case where patterning is performed on a glass substrate by photolithography, the minimum size of a mask pattern is about 2 μm, and the reduction in the channel length of the semiconductor device is restricted by such a size of the mask pattern. The channel length of the semiconductor device is restricted by photolithography, and therefore, is influenced by the in-plane variance of the substrate in the photolithography step.